My colleague was in school one day and gave birth the next. She's terribly active and fit though. Another colleague having her second started leave 2 or 3 weeks before the birth- she was feeling tired and a HOD.

I worked until 38 weeks with my first and he was 2 weeks late, so 4 weeks at home going mad.  I worked until 37+4 with second, was meant to be going until 38+4 but had enough.  She was born 2 days early.  Just rest up at home as much as you can and keep your feet up when possible.  My legs ended up looking like shrek's!

Due to circumstances (supply, moving house, having a toddler to look after too during second pregnancy) I haven't worked past 30 weeks in either pregnancy.  This was, as I say, due to circumstance more than decision but I'm glad to be honest .

I've struggled a lot with pain and general fatigue in pregnancy both times (fatigue probably exacerbated by having to take drowsy anti-emetics till birth to keep the sickness away) and I'm now at 31 weeks struggling to walk much without pain.  I was incredibly sore by 25 weeks in first pregnancy.  First time round I was so uncomfortable, exhausted and sore by 35 weeks I couldn't see how I'd get to the end - but baby dropping down about 37 weeks actually alleviated lots of symptoms and I felt much better for a couple of weeks.

In my pregnancies there is no way I could have worked until 37 weeks, no way at all.  But every pregnancy is different, and every woman (and job, actually!) so you won't necessarily know until you get there. 

You can have one plan and then change it as you go if necessary - take a look at last few pages of the spring babies 2012 or the 'how is your school supporting you' threads to see the balance of people doing fine and people having to take early leave / sick leave. 

 I worked til 38 weeks (39 weeks officially but the last week was half term). I was fine, even if I did spend more time sitting down in the classroom than I normally do. I had a fairly problem free pregnancy, but I was starting to explode out of even my maternity clothes by the end, I was the size of a house!